JAVA8 之 function 基础

r囧r小猫 2023-02-22 15:57 10阅读 0赞

### function 包简介 ###

在java8中新引入了一个包

![20200704165234331.png][]

这是一个重要的更新，牵扯到了lamada表达式和一系列的语法结构变动。

function包主要有六个比较重要的interface；

按照字典排序
    /**
     * Represents an operation that accepts two input arguments and returns no
     * result.  This is the two-arity specialization of {@link Consumer}.
     * Unlike most other functional interfaces, {@code BiConsumer} is expected
     * to operate via side-effects.
     */
    interface BiConsumer<T, U>
    
    /**
     * Represents a function that accepts two arguments and produces a result.
     * This is the two-arity specialization of {@link Function}.
    */
    interface BiFunction<T, U, R>
    
    /**
     * Represents a predicate (boolean-valued function) of two arguments.  This is
     * the two-arity specialization of {@link Predicate}.
     */
    interface BiPredicate<T, U>
    
    /**
     * Represents an operation that accepts a single input argument and returns no
     * result. Unlike most other functional interfaces, {@code Consumer} is expected
     * to operate via side-effects.
     */
    interface Consumer<T> 
    
    /**
     * Represents a function that accepts one argument and produces a result.
     */
    interface Function<T, R> 
    
    /**
     * Represents a predicate (boolean-valued function) of one argument.
     */
    interface Predicate<T>

其实还有几个家伙：如

interface BinaryOperator<T> extends BiFunction<T,T,T>

因为该家伙就是继承的 function接口，把function接口的三个类型(两个入参，一个出参)变成了同一个，所以就不单独说了；

通过翻译软件得知。上面的接口大致的用途如下：

<table style="width:500px;"> 
 <tbody> 
  <tr> 
   <td>接口</td> 
   <td>入参</td> 
   <td>出参</td> 
  </tr> 
  <tr> 
   <td>BiConsumer</td> 
   <td>T、U</td> 
   <td>void</td> 
  </tr> 
  <tr> 
   <td>BiFunction</td> 
   <td>T、U</td> 
   <td>R</td> 
  </tr> 
  <tr> 
   <td>BiPredicate</td> 
   <td>T、U</td> 
   <td>boolean</td> 
  </tr> 
  <tr> 
   <td>Consumer</td> 
   <td>T</td> 
   <td> <p>void</p> </td> 
  </tr> 
  <tr> 
   <td>Function</td> 
   <td>T</td> 
   <td>R</td> 
  </tr> 
  <tr> 
   <td>Predicate</td> 
   <td>T</td> 
   <td>boolean</td> 
  </tr> 
 </tbody> 
</table>

**小结：带BI的多一个入参。**

### 例一： ###

借用上一篇文章的内容，我们在调用Collectors.toMap的时候 最后一个参数内容 为

(u, v) -> v

studentList.stream().collect(Collectors.toMap(Student::getAge, Student::getName, (u, v) -> v));

这里如果使用idea，我们ctrl+鼠标左键可以看到父类就是

interface BinaryOperator<T> extends BiFunction<T,T,T>

![2020070417083566.png][]

因为该方法满足两个入参+一个出参，同时我们看到toMap中的源码如下

public static <T, K, U>
        Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                        Function<? super T, ? extends U> valueMapper,
                                        BinaryOperator<U> mergeFunction) {
            return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
        }

就是使用的

BinaryOperator<U>

那么问题来了，我们怎么知道

(u, v) -> v

哪个是map的旧值，哪个是新值呢？我们到hashmap中的源码查看如下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70][]

发现前面的为旧值，后面的为新值。

### 例二： ###

我们再看一个例子

![202007041713450.png][]

这里的 (k,v) 正好满足我们的BiConsumer ，两个入参，没有出参。我们点进去验证一下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 1][]

foreach的源码入参正好就是biconsumer,和我们的猜想正好吻合。

[20200704165234331.png]: https://img-blog.csdnimg.cn/20200704165234331.png
[2020070417083566.png]: https://img-blog.csdnimg.cn/2020070417083566.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200704171157170.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ==,size_16,color_FFFFFF,t_70
[202007041713450.png]: https://img-blog.csdnimg.cn/202007041713450.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/2020070417150389.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Rvbmd5aW5nMTc1MQ==,size_16,color_FFFFFF,t_70